1. Field
The present invention is directed toward a three-dimensional electrode device and a method of manufacturing such a device. The device may be particularly useful for neuron interface, and more specifically as a cortical implant for vision prosthesis.
2. State of the Art
It is well known that messages are transmitted throughout the nervous system by means of electrical signals. Electrical signals are generated by various parts of the body, such as the sensory organs, and are transmitted to the brain. The brain in turn generates electrical signals to control muscular and other activity. Certain devices have been developed to electrically interface with neural tissue to either receive messages from or deliver messages to the neurons.
For example, various attempts have been made to provide a cortical implant to interface with the region of the cortex corresponding to the sense of sight. Using such implants, blind persons have been made to perceive simple sensations of sight in the form of spots of light, referred to as "phosphenes, " in simple geometric patterns Such interface systems typically include a two-dimensional array of flat electrodes.
These attempts have not been completely satisfactory. Two-dimensional arrays reside on the surface of the cortex. However, the neurons that initiate phosphene perception lie somewhat below the surface. The depth of these neurons is believed to be about 1.5 mm. In surface arrays, relatively high current, in the neighborhood of 3 mA is required to stimulate neurons. Such high currents may pose pathogenic problems. In addition, a phenomenon has been experienced in which when two nearby electrodes are energized, signals from the electrodes interact to produce a phosphene at an anomalous geometric position. Such electrode interactions severely limit the number of electrodes that can be used in surface arrays.
To induce in a blind person the perception of sight, it appears necessary to produce a large number of contiguous phosphenes, similar to the way a cathode ray tube produces a complete image by appropriate illumination of a large number of contiguous "pixels" on a television screen. Because of their construction and limited electrode spacing, previously known implants have not produced the sensation of a sufficient number of contiguous phosphenes to produce an acceptable sense of vision.
Also, some means must be provided to address each of the electrodes individually. One approach has simply been to run a wire to each electrode. With even a small to moderate number of electrodes, such a bundle of wires is cumbersome and disadvantageous, since these wires must lead from the blind person's cerebral cortex to some point external to the blind person's head.
Electrode arrays are also used in applications other than as neuron interfaces. For example, various arrays of photoreceptors or light-emitting diodes are used for image sensing or image producing devices. It is often useful to form such arrays of semiconductor material, particularly silicon, because of a wide variety of characteristics that may be imparted to semiconductors by means of such processes as doping, etching, etc. Such arrays are typically formed of "wafers" of semiconductor material. The electrodes on these wafers are formed by conventional photolithographic techniques. Such wafers may have thicknesses of a millimeter or less, with the electrodes formed on such wafers being in the range of a few microns in thickness.
There remains a need for a three-dimensional semiconductor device that has the capability of providing a large number of electrodes that may be addressed individually for signal transmission and/or reception. Such an array would be particularly advantageous as a neuron interface device, such as a cortical implant for vision prosthesis. Such a three-dimensional array of elongated electrodes may be positioned with the active tips of the electrodes at a depth in the cortex where very localized stimulation of or recording from neurons may more effectively take place. Such an array would preferably be strong and rigid. The array would preferably be formed of a semiconductor material, such as silicon, to make use of the unique electrical properties of semiconductors. The individual electrodes would each be preferably addressable without the need for a large number of lead wires, such as by the provision of a multiplexing system.